1. Field of the Invention
The present invention relates to a method for producing a group III nitride semiconductor light emitting device and a group III nitride semiconductor light emitting device which are suitably used for a light emitting diode (LED), a laser diode, an electronic device, or the like, and also relates to a lamp.
2. Description of the Related Art
A group III nitride semiconductor has a direct transition type band gap of energy that corresponds to a range from the visible light to an ultraviolet region and also has excellent light emission efficiency. For this reason, it is manufactured into a semiconductor light emitting device such as a light emitting diode (LED) and a laser diode (LD), and used in various applications. In addition, the group III nitride semiconductor also has a potential of achieving excellent characteristics when used in electronic devices, as compared to the case where a conventional group III-V compound semiconductor is used.
Such a group III nitride semiconductor is generally produced by a metal organic chemical vapor deposition (MOCVD) method using trimethyl gallium, trimethyl aluminum, and ammonia as source materials. The MOCVD method is a process for growing crystals in which a carrier gas containing the vapor of source materials is delivered to the substrate surface, and the source materials are decomposed on the heated substrate surface.
Conventionally, a single crystal wafer of the group III nitride semiconductor has not been commercially available, and as a method for obtaining group III nitride semiconductors, a commonly used method grows crystals thereof on a single crystal wafer made of different materials. There is a large lattice mismatch between such a substrate made of different materials and the crystals of the group III nitride semiconductor epitaxially grown thereon. For example, when gallium nitride (GaN) is grown on a sapphire (Al2O3) substrate, there is a lattice mismatch of 16% between the two. When gallium nitride is grown on a SiC substrate, there is a lattice mismatch of 6%. In general, when there is a large lattice mismatch as in the above case, it will be difficult to directly grow crystals epitaxially on a substrate, and even when the crystals are grown, crystals with satisfactory crystallinity cannot be attained, which is a problem.
Accordingly, when epucted, in which a layer called a low temperature buffer layer composed of aluminum nitride (AlN) or aluminum gallium nitride (AlGaN) is first laminated on the substrate, and the crystals of the group III nitride semiconductor having a high itaxially growing the crystals of the group III nitride semiconductor on a sapphire single crystal substrate or a SiC single crystal substrate by the metal organic chemical vapor deposition (MOCVD) method, a method has been proposed and has been commonly condtemperature are epitaxially grown thereon (for example, refer to Patent Documents 1 and 2).
However, in the method described in Patent Documents 1 and 2, since there is basically no lattice matching between the substrate and the crystals of the group III nitride semiconductor grown thereon, a dislocation known as threading dislocation which extends towards the surface is incorporated inside the grown crystals. For this reason, distortions of crystals occur, as a result of which sufficient light emission cannot be achieved without the optimization of crystal structure, and the problems such as the decline in the productivity also arise.
On the other hand, in recent years, the use of a material having a crystal structure capable of lattice matching with the crystals of the group III nitride semiconductor as a substrate has been proposed. However, since such a material is reactive with ammonia, Ga, hydrogen, or the like which is used in the MOCVD method as a source material at high temperatures, it has been difficult to grow the crystals, as in the case where a substrate made of sapphire, SiC, or the like is used, by the MOCVD method.
In addition, since inside a reaction apparatus is generally decompressed and gas is circulating therein at an extremely high flow rate, the semiconductor source gas flows into the side surface of the substrate and may even reach the back surface of the substrate. For this reason, there has been a possibility that the above reaction between a substrate and a source material occurs, not only on the front surface of the substrate, but on all of the substrate surfaces which are exposed to the gas containing source materials.
Accordingly, a method has been proposed, in which a buffer layer is formed in advance on a substrate by a method other than the MOCVD method, followed by the introduction of this substrate on which the buffer layer is formed to an MOCVD reaction furnace (for example, refer to Patent Documents 3 and 4). In the method described in Patent Documents 3 and 4, the buffer layer is formed on the substrate through a reactive sputtering process, while as a substrate material, sapphire, silicon, silicon carbide, zinc oxide, gallium phosphide, gallium arsenide, magnesium oxide, manganese oxide, single crystals of a group III nitride based semiconductor, or the like, is used, and among them, a sapphire a-plane substrate is particularly suitable.
According to the above Patent Documents 3 and 4, since the buffer layer is favorably oriented due to the formation of the buffer layer by the process employing a reactive sputtering method, the crystallinity of the group III nitride semiconductor formed thereon is improved.    [Patent Document 1] Japanese Patent Publication No. 3026087    [Patent Document 2] Japanese Unexamined Patent Application, First Publication No. Hei 4-297023    [Patent Document 3] Japanese Patent Publication No. 3440873    [Patent Document 4] Japanese Patent Publication No. 3700492
However, as a result of intensive and extensive studies, the present inventors and others discovered that when a buffer layer made of the above materials is formed on the substrate surface using the method described in Patent Documents 1 and 2, there are limits in order to further improve the crystallinity of the gallium nitride based compound semiconductor formed thereon using the MOCVD method. It is assumed that this problem is due to the inclusion of amorphous and/or polycrystalline phases in the buffer layer when the method described in Patent Documents 1 and 2 is employed. Similarly, in a film forming method, which uses aluminum nitride deposited through a sputtering process as a buffer layer, like the method described in Patent Documents 3 and 4, there has been a problem in that no further improvements in the crystallinity can be achieved due to the difference between the lattice constant of the buffer layer and that of the gallium nitride layer.